1. Technical Field
The present invention relates to a system and a method for controlling bit rate of a video signal and an audio signal.
2. Background Art
A video and audio bit rate control system has been developed. The system receives a video signal and an audio signal such as a movie and news from a broadcasting station, converts the signals into a form applicable to a digital television set in home, and performs signal multiplexing control of these converted signals by statistical multiplexing scheme for transmitting to the digital television sets in home.
The video and audio bit rate control system includes an encoder which encodes the video signal and the audio signal such as a movie and news.
A bit rate of signal to be encoded is assigned to the encoder according to a type of signals such as the video signal, the audio signal and the other data signal. Here, the bit rate corresponding to each signal is called as a video bit rate, an audio bit rate or the other data bit rate. The variable biterate is assigned for the video signal, and the fixed bit rate is assigned for the audio signal in advance. The above mentioned “bit rate to be assigned” means “a bit rate which the encoder can output”, that is, a maximum bit rate which the encoder may output. Accordingly, a total of the video bit rate, the audio bit rate and the other data signal bit rate corresponds to a maximum total bit rate which the encoder may output. The encoder encodes each of the video signal, the audio signal and other data signal so as not to exceed each maximum bit rate which corresponds to each assigned bit rate according to a type of the signals.
Moreover, the encoder encodes the audio signal so that the bit rate of the encoded audio signal may not exceed the audio bit rate variably instructed by an encoder control device which controls the encoder.
Thus, the encoder does not use up a biterate which is reserved as the audio bit rate in advance, when the audio bit rate instructed by the encoder control device is lower than the audio bit rate which is assigned fixedly in advance (i.e. maximum available bit rate). As a result, the bit rate which is not used up is wasted.
Above situation will be explained in the following, with reference to FIG. 6. FIG. 6 is a schematic diagram showing an example of time variation of the video bit rate and the audio bit rate which are assigned to the encoder in the above mentioned video and audio bit rate control system. Moreover, FIG. 6 also shows the other data bit rate which is assigned fixedly.
A horizontal axis of FIG. 6 indicates time course. A vertical axis of FIG. 6 indicates the bit rate to be handled by the encoder. As shown in FIG. 6, the video bit rate is assigned variably, the audio bit rate is assigned fixedly and the other data bit rate is assigned fixedly, and a total addition of these bit rates corresponds to a maximum encoder bit rate which the encoder can output. In these circumstances, the encoder performs each encoding of the video signal, the audio signal and the other data signal using respective assigned bit rates. When, at a time point indicated by “INSTRUCTION TO CHANGE AUDIO BITRARE” in FIG. 6, the encoder control device instructs to change the bit rate for encoding the audio signal using a new bit rate which is lower than the pre-assigned bit rate up to that time, the encoder commences the encoding of the audio signal using the new bit rate instructed by the encoder control device from that time point.
Although the new bit rate for encoding the audio signal is lower than the pre-assigned bit rate which is reserved for the audio signal, the encoder encodes the video signal and the other data signal in a similar way as before. This means that the encoder does not use the bit rate corresponding to a difference between the new bit rate and the pre-assigned bit rate, and this unused bit rate is wasted.
Japanese Patent Application Laid-Open No. 1995-202820 discloses a bit rate control system which controls a bit rate dynamically. The system includes an audio encoder, a video encoder, a bit rate assigning device, a FIFO buffer and a system multiplex device. The audio encoder encodes an audio signal and outputs an encoded audio signal bit stream. Moreover, the audio encoder outputs a total bit number saved by the audio encoder. The video encoder receives a video signal, encodes the input video sequence at a rate of fixed number of video units every second and outputs a video bit stream. The bit rate assigning device dynamically assigns a total of bits, which is usable to encode the present video signal, to the video encoder using all the bit number saved by the audio encoder. The FIFO buffer buffers the encoded audio bit stream and the encoded video bit stream and outputs the audio bit stream and the video bit stream at a constant bit rate. The system multiplex device multiplexes the audio bit stream and the video bit stream which are output by the FIFO buffer.
As mentioned above, the bit rate control system described in Japanese Patent Application Laid-Open No. 1995-202820 adopts a single encoder (voice encoder and video encoder). The single encoder dynamically assigns the saved bit rate out of the assigned audio bit rate to the video signal.
Japanese Patent Application Laid-Open No. 2002-057638 discloses a multiplex apparatus which processes a plurality of video signals and a plurality of audio signals which are inputted thereto. The multiplex apparatus described in Japanese Patent Application Laid-Open No. 2002-057638 includes a source encoding unit, a priority decision unit and a media multiplex unit. The source encoding unit includes a plurality of video encoders, audio encoders and data encoders. The source encoder receives a media signal. The media signal includes a plurality of video signals, a plurality of audio signals, various data signals and contents information which indicates contents of each signal. Each encoder, which is included in the source encoding unit, encodes the video signal, the audio signal and various data signals to output an encoded video bit stream, an encoded audio bit stream or an encoded data bit stream. The priority decision unit decides priority of the media signal based on the contents information, which is included in the media signal received by the source encoding unit, with reference to order which is decided in advance according to the signal contents. The media multiplex unit multiplexes each encoded bit stream which the source encoding unit outputs according to the priority of media information.
The multiplex apparatus described in Japanese Patent Application Laid-Open No. 2002-057638 encodes a plurality of video signals, a plurality of audio signals and various data signals, and outputs respective encoded bit streams. As the multiplex scheme, the media multiplex unit multiplexes each encoded bit stream according to the order of priority which is decided in advance based on the contents of the signals.